Self-driven synchronous rectification and voltage stabilization circuit

ABSTRACT

A synchronous rectification and voltage stabilization circuit includes a saturable inductance coil (L 1 ), a synchronous rectifier switch ( 33 ), a driving coil (L 2 ) for turning on the synchronous rectifier switch, a discharging coil (L 3 ) which co-operates with a controllable switching component (Q 1 ) to turn off the synchronous rectifier switch, a filter circuit ( 40 ) and a voltage stabilization circuit ( 50 ). The saturable inductance coil, the synchronous rectifier switch and the filter circuit are connected in serial between an input terminal ( 10 ) and an output terminal ( 20 ). The saturable inductance coil is coiled together with the driving coil and the discharging coil on one core. The voltage stabilization circuit is connected between the output terminal and the saturable inductance coil, and is used for regulating a regulable saturation point of the saturable inductance coil according to an output voltage of the output terminal and a target value of the output voltage.

TECHNICAL FIELD

The present invention relates to synchronous circuits, and particularlyto a self-driven synchronous rectification and voltage stabilizationcircuit.

RELATED ART

There is an ever-increasing demand in the power electronics market forlow voltage and high current DC-DC converters. As an output voltage isdesired to be 3.3 V or lower, even with a state-of-the art Schottkydiode with a forward voltage drop of 0.3 V has an unacceptable amount ofpower loss.

Because of this, synchronous rectifiers are often used to improve theefficiency of DC-DC converters. Generally, there are two types ofsynchronous rectifiers, self-driven and externally driven. Since theself-driven mode is usually less complex, less costly, and morereliable, it is a preferred choice for use with most low voltage DC-DCconverter applications.

In a conventional self-driven synchronous rectification DC-DC converter,in order to regulate an output voltage thereof, a Pulse Wave Modulation(PWM) control circuit is typically employed. The PWM control circuitoutputs PWM waves according to a feedback from the output voltage. ThePWM waves switch a power switch connected with a primary winding of atransformer on and off, thus regulating the output voltage. However, insuch a regulation mode, for down-stream components of the PWM controlcircuit such as the transformer are uncontrollable, the output voltagecannot be exactly regulated to its target value.

In addition, in the conventional self-driven synchronous rectificationcircuit, a secondary winding of a main transformer is employed todirectly turn on a synchronous rectifier switch, while a dischargingcircuit is employed to quickly turn off the synchronous rectifierswitch. The discharging circuit typically includes a discharging coilthat produces an inducted voltage thereaccross according to a currentchange occurred in the main transformer or another transformer. However,the discharging coil is usually coiled on a core different from the maintransformer core. This not only brings down the input/output ratio, butalso increases a size of the self-driven synchronous rectificationcircuit.

Therefore, there is a need for providing a synchronous rectification andvoltage stabilization circuit which can solve the above-mentionedproblems.

SUMMARY

A synchronous rectification and voltage stabilization circuit isprovided in accordance with a preferred embodiment. The synchronousrectification and voltage stabilization circuit includes a synchronousrectification circuit and a filter circuit connected. The synchronousrectification circuit further includes a synchronous rectifier switch, adriving circuit comprising a driving coil for turning on the synchronousrectifier switch, and a discharging circuit comprising a dischargingcoil and a controllable switching component for co-operating to turn offthe synchronous rectifier switch. The synchronous rectification circuitis connected in serial with the filter circuit between an input terminaland an output terminal. The synchronous rectification and voltagestabilization circuit further includes a saturable inductor and avoltage stabilization circuit. The saturable inductor is connectedbetween the input terminal and the synchronous rectifier switch andincludes a saturable inductance coil. The saturable inductance coil iscoiled together with the driving coil and the discharging coil on onecore, and has a regulable saturation point. The voltage stabilizationcircuit is connected between the output terminal and the saturableinductance coil, and is used for regulating the regulable saturationpoint of the saturable inductance coil according to an output voltage ofthe output terminal and a target value of the output voltage.

Other advantages and novel features will be drawn from the followingdetailed description with reference to the attached drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an exemplary block diagram of a self-driven synchronousrectification and voltage stabilization circuit in accordance with apreferred embodiment of the present invention;

FIG. 2 depicts a circuit diagram of the self-driven synchronousrectification and voltage stabilization circuit of FIG. 1;

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to FIG. 1, a block diagram of a self-driven synchronousrectification and voltage stabilization circuit mainly includes asaturable inductor 20, a synchronous rectification circuit 30, a filtercircuit 40, and a voltage stabilization circuit 50. The saturableinductor 20, the synchronous rectification 30 and the filter circuit 40are sequentially connected in serial between an input terminal 10 and anoutput terminal 60. The saturable inductor 20 receives an AC power froman AC power source (not shown) via the input terminal 10 and forwardsthe AC power to the synchronous rectification circuit 30. Thesynchronous rectification circuit 30 and the filter circuit 40co-operates and converts the AC power to a DC power and then outputs theDC power to the output terminal 60. The AC power may be a unidirectionalrectangular wave power. The unidirectional rectangle wave power may beresulted from a secondary DC power after being switched on and off by apower switch. The voltage stabilization circuit 50 is connected betweenthe output terminal 60 and the saturable inductor 20. The voltagestabilization circuit 50 regulates a regulable saturation point of thesaturable inductor 20 according to an output voltage of the outputterminal 60, thus regulating the output voltage to a target value.

The synchronous rectification circuit 30 mainly includes a synchronousrectifier switch 33, a driving circuit 31 and a discharging circuit 32.The synchronous rectifier switch 33 is connected between the saturableinductor 20 and the filer circuit 40. The driving circuit 31 and thedischarging circuit 32 are connected in parallel between the saturableinductor 20 and a gate of the synchronous rectifier switch 33. Thedriving circuit accumulates charges on the gate of the synchronousrectifier switch 33, thus switching on the synchronous rectifier switch33. The discharging circuit 32 acts contrary to the driving circuit 31;the discharging circuit removes the charges that the driving circuitaccumulates from the gate of the synchronous rectifier switch 33, thusswitching off the synchronous rectifier switch 33.

The voltage stabilization circuit 50 mainly includes a saturableinductor controlling circuit 51 and a reference voltage providingcircuit 52. The reference voltage providing circuit 52 is used toproduce a reference voltage for the saturable inductor controllingcircuit 51. In one embodiment as shown in the FIG. 1, the referencevoltage providing circuit 52 produces the reference voltage according tothe output voltage of the output terminal 60.

However, in an alternative embodiment, the reference voltage providingcircuit 52 is disconnected from the output terminal 60, allowing thereference voltage providing circuit 52 to be controlled by othercomponents to produce the reference voltage, or to produce the referencevoltage at a constant value that corresponds to the target value of theoutput voltage independent of other components. The saturable inductorcontrolling circuit 51 is connected between the output 60 and thesaturable inductor 20. The saturable inductor receives the referencevoltage from the reference voltage providing circuit 52 and regulatesthe regulable saturation point of the saturable inductor 20 according tothe reference voltage.

Referring to FIG. 2. The saturable inductor 20 includes a saturableinductance coil L1. The driving circuit 31 includes a driving coil L2,while the discharging circuit 32 includes a discharging controlling coilL3 and a controllable switching component; for example, a switching NPNtransistor Q1. The saturable inductance coil L1, the driving coil L2,and the discharging circuit 32 are coiled round a core.

A dotted terminal of the saturable inductance coil L1 is connected tothe input terminal 10 and an undotted terminal of the saturableinductance coil L1 is connected to the synchronous rectifier switch 33.A dotted terminal of the driving coil L2 is connected to the gate of thesynchronous rectifier switch 33 via a resistance component (e.g., aresistor R1), and an undotted terminal of the driving coil L2 isconnected to the undotted terminal of the saturable inductance coil L1.A dotted terminal of the discharging controlling coil L3 is connected tothe dotted terminal of the saturable inductance coil L1, and an undottedterminal of the discharging coil is connected to a base of the switchNPN transistor Q1 via a resistance component (e.g., a resistor R2). Theswitch NPN transistor Q1 is connected with the gate of the synchronousrectifier switch 33 at a collector thereof, and connected with theundotted terminal of the saturable inductance coil L1 at an emitterthereof.

A change of a current through the saturable inductance coil L1 resultsin corresponding changes of voltages across the driving coil L2 and thedischarging coil L3. When the current through the saturable inductancecoil L1 changes from a high level to a low level, an inducted voltage(hereinafter “the inducted voltage I”) is produced across the drivingcoil L2. The inducted voltage I accumulates charges on the gate of thesynchronous rectifier switch 33 and therefore turns on the synchronousrectifier switch 33. When the current through the saturable inductancecoil L1 changes from the low level to the high level, an inductedvoltage (hereinafter “the inducted voltage II”) is produced across thedischarging coil L3, The inducted voltage II turns on the switching NPNtransistor Q1, thus to move the charges from the gate of the synchronousrectifier switch 33. The synchronous rectifier switch 33 is thereforeturned off.

The filter circuit 40 shown in is a pi low pass filter constituted bytwo capacitors C1 and C2, and an inductor L4. The inductor L4 isconnected between the synchronous rectifier switch 33 and the outputterminal 60. The capacitors C1 and C2 are interposed on each side of theinductor L4. Each of the capacitors C1 and C2 has two terminals, ofwhich one is connected with the inductor L4 and another is connectedwith the ground potential. The filter circuit 40 may alternativelyemploy other types of filters, such as an L-type low pass filter.

The saturable inductor controlling circuit 51 mainly includes a voltagestabilizer W1 and a controllable switching unit. The voltage stabilizerW1 is used to keep a steady voltage (e.g., a zener voltage) thereacross.The steady voltage corresponds to the target value of the outputvoltage. In FIG. 2, the voltage stabilizer W1 is shown as a controllablethree-terminal voltage stabilizer including a controlling terminal andtwo conducting terminals. One of the conducting terminals is connectedwith the controllable switching component, and another of the conductingterminals is connected with the ground potential. Currents arecontrolled by the controlling terminal to flow through the conductingterminals.

The controllable switching unit is shown in FIG. 2 as a switching PNPtransistor Q2 having a base, a collector, and an emitter. The base ofthe switching PNP transistor is connected with one conducting terminalof the controllable three-terminal voltage stabilizer. The collector ofthe switching PNP transistor Q2 is connected with the output terminal 60via a resistance component, such as a resistor R3 shown in FIG. 2. Theemitter of the switching PNP transistor Q2 is connected with theundotted terminal of the saturable inductance coil L1 via a diode D1.The diode D1 permits currents to flow from the switching PNP transistorQ2 to the saturable inductance coil L1 and thus to regulate theregulable saturation point of the saturable inductance coil L1. Further,a diode D2 is connected between the emitter of the switching PNPtransistor Q2 and the ground potential.

The reference providing circuit 52 shown in FIG. 2 is a voltage dividingcircuit. The voltage dividing circuit is connected between the outputterminal 60 and the ground potential and includes two serial-connectedresistors R4 and R5. The resistors R4 and R5 forms a node Atherebetween, and the node A is connected to the base of the switchingPNP transistor Q2 via a capacitor C3, and to the controlling terminal ofthe controllable three-terminal voltage stabilizer.

The voltage dividing circuit and the controllable three-terminal voltagestabilizer co-operates to turn on the switching PNP transistor Q1, thusto feed back a suitable amount of current to the saturable inductor 20.In this embodiment, the suitable amount of current is determined by thevoltage dividing circuit and the controllable three-terminal voltagestabilizer according to the output voltage and the target value of theoutput voltage.

It is believed that the present embodiments and their advantages will beunderstood from the foregoing description, and it will be apparent thatvarious changes may be made thereto without departing from the spiritand scope of the invention or sacrificing all of its materialadvantages, the examples hereinbefore described merely being preferredor exemplary embodiments of the invention.

1. A synchronous rectification and voltage stabilization circuit,comprising a synchronous rectification circuit and a filter circuitconnected in serial between an input terminal and an output terminal,wherein: the synchronous rectification and voltage stabilization circuitfurther comprises: a saturable inductor connected between the inputterminal and the synchronous rectification, the saturable inductorhaving a regulable saturation point; and a voltage stabilization circuitconnected between the output terminal and the saturable inductor, thevoltage stabilization circuit being configured for regulating theregulable saturable point of the saturable inductor according to anoutput voltage of the output terminal and a target value of the outputvoltage.
 2. The circuit as claimed in claim 1, wherein the saturableinductor comprises a saturable inductance coil.
 3. The circuit asclaimed in claim 2, wherein the synchronous rectification circuitcomprises a driving circuit, a discharging circuit and a synchronousrectifier switch, the driving circuit comprising a driving coil, thedischarging circuit comprising a discharging coil and a controllableswitching component, and the synchronous rectifier switch comprising agate.
 4. The circuit as claimed in claim 3, wherein the saturableinductance coil, the driving coil and the discharging coil are coiledround one core, and each has a dotted terminal and an undotted terminal.5. The circuit as claimed in claim 4, wherein the saturable inductancecoil is connected with the input terminal at the dotted terminalthereof, and connected with the synchronous rectifier switch at theundotted terminal thereof, the driving coil is connected with the gateof the synchronous rectifier switch at the dotted terminal thereof, andconnected with the undotted terminal of the saturable inductance coil atthe undotted terminal thereof, and the discharging coil is connectedwith the undotted terminal of the saturable inductance coil at thedotted terminal thereof, and connected with the controllable switchingcomponent at the undotted terminal.
 6. The circuit as claimed in claim5, wherein a change of a current through the saturable synchronous coilfrom a high level to a low level produces a first inducted voltageacross the driving coil, and the inducted voltage turns on thesynchronous rectifier switch.
 7. The circuit as claimed in claim 6,wherein a change of the current through the saturable synchronous coilfrom the low level to the high level produces a second inducted voltageacross the discharging coil, and the second inducted voltage controlsthe controllable switching component to conduct and therefore turns offthe synchronous rectifier switch.
 8. The circuit as claimed in claim 1,wherein the voltage stabilization circuit comprises a saturable inductorcontrolling circuit and a reference voltage providing circuit, thereference voltage producing a reference voltage according to the outputvoltage or the target value of the output voltage, and the saturablefeeding back a suitable amount of current to the saturable inductoraccording to the output voltage and the target value of the outputvoltage.
 9. A synchronous rectification and voltage stabilizationcircuit, comprising a synchronous rectifier switch, a driving circuitcomprising a driving coil for turning on the synchronous rectifierswitch, a discharging circuit comprising a discharging coil and acontrollable switching component for co-operating to turn off thesynchronous rectifier switch, and a filter circuit connected with thesynchronous rectifier switch between an input terminal and an outputterminal, wherein: the synchronous rectification and voltagestabilization circuit further comprises: a saturable inductor comprisinga saturable inductance coil connected between the input terminal and thesynchronous rectifier switch, the saturable inductance having aregulable saturation point and being coiled together with the drivingcoil and the discharging coil round one core; and a voltagestabilization circuit connected between the output terminal and thesaturable inductance, the voltage stabilization circuit being configuredfor regulating the regulable saturable point of the saturable inductanceaccording to an output voltage of the output terminal and a target valueof the output voltage.
 10. The circuit as connected in claimed 9,wherein the synchronous rectifier switch comprises a gate.
 11. Thecircuit as claimed in claim 10, wherein the saturable inductance coil,the driving coil and the discharging coil each has a dotted terminal andan undotted terminal, the saturable inductance coil being connected withthe input terminal at the dotted terminal thereof, and being connectedwith the synchronous rectifier switch at the undotted terminal thereof,the driving coil being connected with the gate of the synchronousrectifier switch at the dotted terminal thereof, and being connectedwith the undotted terminal of the saturable inductance coil at theundotted terminal thereof, and the discharging coil being connected withthe undotted terminal of the saturable inductance coil at the dottedterminal thereof, and being connected with the controllable switchingcomponent at the undotted terminal.
 12. The circuit as claimed in claim11, wherein a change of a current through the saturable inductance coilfrom a high level to a low level produces a first inducted voltageacross the driving coil and the first inducted voltage turns on thesynchronous rectifier switch.
 13. The circuit as claimed in claim 12,wherein a change of the current through the saturable inductance coilfrom the low level to the high level produces a second inducted voltageacross the discharging coil and the second inducted voltage controls thecontrollable switching component to turn off the synchronous rectifierswitch.
 14. The circuit as claimed in claim 9, wherein the voltagestabilization circuit comprises a saturable inductor controlling circuitand a reference voltage providing circuit, the reference voltageproducing a reference voltage according to the output voltage or thetarget value of the output voltage, and the saturable feeding back asuitable amount of current to the saturable inductor according to theoutput voltage and the target value of the output voltage.